April, 1911.] The Ancient Vegetation of Ohio. 327 



whatever nature checked the activity of the roots of plants and 

 depressed their transpiration. The strildng similarity of the 

 aerial shoots of the carboniferous plants to those of modem times 

 in bogs and undrained swamps restrain one, therefore, from 

 assuming that the atmosphere differed greatly in temperature and 

 humidity, or was different in the chemical constituents from what 

 it is now. There may have been moderate variations in the 

 carbon dioxide content of the air, but this would require experi- 

 mental proof upon bog plants and the groups of plants similar to 

 those which lived in carboniferous times, the scouring rushes, the 

 lycopods, ferns, cycads and gymnosperms, to assign its limits. 

 The statements in current literature as to the strengths of that 

 gas which green plants can endure are conflicting (G), and call for 

 further work in the field and in the laboratory. 



The consideration of these facts leads to another point — the 

 inevitable conclusion that the form characters and the funda- 

 mental resistance to drought and dessication distinctive of xero- 

 phytic plants whether in bogs or deserts must have made their 

 appearance within early geologic time. They are not of recent 

 development (15). The climate of northern America has under- 

 gone oscillations between periods of maximum aridity and max- 

 imum precipitation and humidity, with extreme variations in 

 temperature during and following the several glacial periods; the 

 amplitude occupying periods of perhaps many thousands of 

 years. Variations in climate so wide apart indicate an almost 

 complete change in the character of the flora during the geologic 

 periods. The xerophytic features which characterize bogs and 

 deserts are not to be taken, therefore, as having come about by a 

 direct and continuously increasing edaphic or climatic aridity. 

 Aside from the question as to the methods and the activating 

 conditions in evolutionary development, it seems certain that the 

 origin of xeroph}' tic forms is not one of recent development in 

 the vegetable kingdom but must have been concomitant with the 

 diastrophic and gradation processes of the great geologic periods. 

 The great floral evolutions of geologic history were principally 

 one of growth-form, physiognomy, and functional behaviour, and 

 not of floral structure alone. Water has always been the most 

 important of all the life relations in the environment of plants. 

 In the early tyjoes of gametophytic vegetation it remained neces- 

 sarily of greatest importance for the movements of gametes in 

 effecting fertilization and for dissemination. The luxurious 

 development of these fonns in the ancient areas of low lying land 

 became checked in the stress of aridity encountered with the 

 accumulation of their debris. With the origin and the develop- 

 ment of the sporophytic types of vegetation, which were from the 

 first less dependent upon free water, the prolongation of vegeta- 

 tion activity enabled the plants to occupy the areas with greater 



